1. Field of the Invention
The present invention is directed to methods and apparatus for measuring a ratio of a variable for assessment in medical imaging data of a subject, in particular in MR and PET imaging data.
2. Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions. Other such functional imaging modalities are known, such as SPECT. Other modalities for example are Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), known as anatomical imaging modalities.
In functional images, many important pathologies and anatomical structures appear as very high (or low) intensities. For example, a tumor in an FDG-PET image will often appear as a bright region.
For some types of medical image analysis, for example assessment of PET-florbetapir images for amyloid plaque deposition, it is useful to be able to quantify the ratio of uptake between areas of (specific) grey matter and (unspecific) white matter in the brain in a local neighborhood.
The most basic approach to solving this problem is to look visually at the image. However, problems of consistency can arise when different readers view the image, and in particular changing the window and level used to view the data can greatly affect the interpretation.
Alternatively, a user can manually create first and second regions of interest (ROIs), one in each part of the image to be quantified, and then calculate the ratio of the mean uptake in each of the regions. This approach provides quantification, but is time-consuming, to perform, especially if multiple readings from different areas are required, as each reading requires interaction with the ROIs. In addition, if the areas to be investigated are small or thin (like the cortical grey matter), the ROIs can be hard to position precisely.